In humans and other animals, normal body function involves many energy consuming activities. A great deal of energy is consumed by bodily locomotion such as walking and running. However, most of the mechanical energy used for these functions is not reused, but is instead returned to the environment as thermal energy.
A body energy harvesting system, for example, can harvest mechanical energy from body movements and transform it into electrical energy. Since most body movements (e.g., human movements) are unpredictable, body energy harvesting systems have generally been restricted to capturing energy from regular movements in certain activities at the cost of increased metabolic activity of the subject.